1. Field of the Invention
The present invention relates to a device and a method for writing on imaging material. The device includes a radiation source for generating an electromagnetic radiation of such a wavelength that image information can be written to the imaging material using the wavelength. A waveguide is integrated with a substrate material and arranged as a modulator for modulating the radiation produced by the radiation source. The modulator includes an input capable of receiving a modulation signal which includes the image information.
2. Description of the Related Art
A device and a method of this type are described in the published international patent application WO 96/25009. This application describes a system for producing color images for rendering color or monochrome images, in particular for television and video applications and for printing, for example for printing on photo sensitive paper. The disclosed system for generating images includes waveguides which are integrated on a substrate in either monolithic or hybrid form. These integrated waveguides are implemented as modulators for modulating the intensity of the light guided in the waveguides with a modulation signal having wavelengths in the visible region of the spectrum. The modulated light exiting from the waveguide is transmitted to a device for deflecting the beam, such as a polygon mirror, which deflects the modulated lights to a photo sensitive surfacexe2x80x94for example, photographic paperxe2x80x94so that the photo sensitive surface can be written line by line. They patent application also discloses that light of three wavelengths in the red, green and blue wavelength region of the visible spectrum are modulated with an integrated modulator and that the three modulated light beams are subsequently superimposed to form a color image. The disclosed image generating system therefore represents a color mixer capable of generating any desired color on the photo sensitive surface. Preferably, potassium titanyl phosphate, KTiOPO4, (KTP) is used as substrate material for the color mixer. This substrate material is particular suited for guiding single-mode the radiation in the entire spectral range of the visible light.
The integrated optical waveguide disclosed in WO 96/25009 is investigated in more detail in the dissertation xe2x80x9cUntersuchung der physikalischen Eigenschaften ionenausgetauschter optischer Wellenleiter und Wellenleiterbauelemente in KTPxe2x80x9d by J.-P. Ruske, Department for Physical, Astronomical and Technical Sciences of the Friedrich-Schiller University, Jena. The dissertation in particular discusses advantageous and disadvantageous waveguide properties. The dissertation investigates various effects causing a change in the effective index of refraction of the modes guided in the waveguide and change in the spontaneous polarization of the substrate material. The changes in the index of refraction and polarization cause changes in the phase of the radiation guided in the waveguide. Such phase changes are caused, in particular, by thermal, photo refractive and pyroelectric effects due to the optical power of the optical radiation guided in the waveguide. The absorption of the substrate material converts the guided light energy is into heat, causing the temperature of the waveguide region to increase. This heating then causes that changes in the index and refraction and polarization mentioned above. In addition, strain is produced in the substrate material, which also affects the phase of the guided radiation through photo elastic interactions. The changes in the index of refraction caused by the photo refractive effect are directly induced by the light. The effect of the investigated parameters on the waveguide properties is dependent on the power of the guided radiation. The dissertation, however, comes to the conclusion that the changes in the refractive index and phase do not significantly affect the operation of the integrated optical waveguides.
Based on the teaching of the WO 96/25009, it is the object of the present invention to improve the rendition of image information on an imaging material.
The present invention is based on the observation that writing on imaging material with an integrated waveguide implemented as a modulator poses particularly stringent requirements. The term xe2x80x9cimaging materialxe2x80x9d includes many different materials suitable for rendering image information. For example, imaging materials can be recording materials for permanently recording image information, such as photographic paper, photographic or thermographic film or selenium drums, and projection materials, for example for television and video applications. Imaging materials have different properties, such as a specific sensitivity with respect to the absolute energy of a radiation, which can be used to generate on the imaging material a desired representation of image information. In particular with photographic recording material, the maximum and minimum attainable density of the recording material is predetermined by the recording material itself, as well as the resolvable density steps between that minimal and maximal density. The modulated radiation for recording should therefore advantageously be matched to the respective imaging material.
With the invention, modulation errors which occur when they radiation used to write on the imaging material is modulated, can be compensated or at least significantly reduced, so that image information can advantageously be rendered on imaging material with vibrant highlights and in high resolution. Modulation errors can be caused, for example, when the substrate material heats up. Other factors, for example, an unwanted rotations of the polarization in the radiation source itself, i.e., at the time the electromagnetic radiation is generated or when the supplied radiation is guided in the modulatorxe2x80x94e.g., in a light waveguidexe2x80x94can also cause modulation errors. In any event, the modulation errors reduce the quality with which the image information is rendered on the imaging material as compared to the desired rendition of the image information.
According to an advantageous embodiment of the invention, a modulation errors signal is produced and used by the compensatior for compensating the modulation error. This modulation errors signal is produced by comparing a set-point information with an image of the radiation modulated by the modulator. The so determined modulation error can advantageously be used to compensate the modulation error more accurately. The set-point information may be predetermined and selected to allow a meaningful comparison with the image of the modulated radiation corresponding to an optimized modulation. The set-point information may, for example, include image information that is to be written on the imaging material.
According to another advantageous embodiment, an image signal is supplied to the compensator, wherein the image signal contains image information and the compensator generates the modulation signal in dependence of the image signal and the modulation error signal. In this way, the modulation error signal to be used to compensate the modulation error can be combined directly in the compensator with the image information This makes the control of the modulator with a modulation signal which includes the image information to be written on the imaging material, and the control information for compensating the modulation error less complex.
According to yet another advantageous embodiment of the invention, the device includes a evaluation means for processing the spectral composition of the modulated radiation. In this way, different error causes responsible for the modulation errors, in particular modulation errors at different frequencies, can be analyzed. The spectral composition of the modulated radiation can be analyzed simply by analyzing the image of the modulated radiation. To minimize distortion and other errors which can be introduced during an analysis of the spectral composition of modulated radiation, the evaluation means can advantageously be integrated directly in the modulator in the substrate material.
Advantageously, the information about the spectral composition of the modulated radiation is used to generate the modulation error signal. In this way, all or only a limited number of error causes responsible for producing the modulation error can be selectively incorporated directly into the modulation errors signal, so that these error causes can be compensated by properly correcting the error via control of the modulator.
It yet another advantageous embodiment of the invention, the compensator includes a temperature stabilizor acting on the modulator and stabilizing its temperature. In this way, modulation errors caused by heating of the modulator can be reduced or compensated entirely. With the temperature stabilizer, the temperature of the modulator is adjusted to a value which optimizes the desired operation. The temperature stabilizer may, for example, advantageously be controlled with a suitable signal which defines the stabilization temperature for the modulator. The stabilization temperature can be more accurately determined by analyzing the low-frequency components of the modulated radiation which were determined when the spectral composition was analyzed. The upper frequency limit of the low-frequency components depends on the heat diffusion velocity of the substrate material in which the modulator is integrated. This advantageous embodiment is based on the observation that temperature effects affecting the modulator in particular cause modulation errors which can be deduced from the low-frequency components of the modulated radiation. Such temperature related modulation errors may be caused by temperature drift having a duration in the range of seconds.
In order to compensate modulation errors caused by such temperature effects with high accuracy, a sensor can be provided for determining the temperature of the modulator. The so determined temperature of the modulator can be used to produce the control signal for controlling the temperature stabilizer. In this way, the temperature of the modulator required for proper modulation can be adjusted more precisely. Alternatively or in addition, the temperature determined with the sensor can also be used to generate the modulation error signal. Modulation errors caused by temperature errors can thereby be compensated directly by controlling the modulator with the modulation signal.
In yet another advantageous embodiment of the invention, an image of the unmodulated radiation is captured. A second capture means for capturing this image can be arranged, for example, before the modulator, so that the image of the radiation is captured before the radiation produced by the radiation source is modulated in the modulator by the modulation signal. Alternatively, the second capture means may also be located after the modulator so that the image of the un-modulated radiation is captured when the modulator is not in operation, , when the radiation produced by the radiation source passes through the modulator without being modulated. The modulator is here in a passive state. Advantageously, the image of the unmodulated radiation can also be captured by the first capture means arranged after the modulator, which radiation is otherwise used to capture an image of the modulated radiation. To minimize the technical complexity for capturing the images of the radiation, it is also possible to capture the modulated radiation, when the modulator is operating, and the un-modulated radiation when the modulator is passive, using a single capture means. The captured image of the un-modulated radiation can be used to determine unwanted properties of the radiation which can cause modulation errors during modulation in the modulator, before the radiation is modulated with the modulation signal.
The captured image of the un-modulated radiation can then be supplied to the compensator for adjusting the radiation before the radiation is passed on to the modulator to modulate the radiation with the modulation signal. In this way, the radiation can be modified before the actual modulation process to eliminate modulation errors during the modulation. Alternatively or in addition, the image of the un-modulated radiation supplied to the compensator can be used to generate the modulation signal by directly controlling the modulator with a suitable modulation signal in order to compensate or at least reduce the modulation errors. Advantageously, the polarization of the radiation can be adjusted with a polarizer before the radiation is supplied to the modulator for modulation. This approach is particularly advantageous if the polarization direction of the radiation supplied to modulator does not correspond to the polarization direction to which the modulator is adjusted. In this way, the greatest possible modulation depth between a maximum in the intensity and total extinction of the modulated radiation can be attained.
Additional advantageous embodiment of the invention are recited in the dependent claims.
The invention and its advantages will be described hereinafter with reference to specific embodiments.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are intended solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.